The present invention generally relates to communications systems and, more particularly, to a communications systems having a plurality of code rates and modulation schemes.
Digital Video Broadcasting-Cable (DVB-C) is a first generation cable transmission system (e.g., see EN 300 429 V.1.2.1 (1998-04) Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for cable systems). However, migration to a second generation cable system, DVB-C2, is currently being studied for increasing transport capacity in order to offer new services such as high-definition television (HDTV), Video-on-Demand (VoD), and other advanced personalized and interactive services in the cable environment.
One way of improving performance in a second generation cable system is to adopt for use in DVB-C2 the low density parity check (LDPC) forward error correction codes used in the second generation of Digital Video Broadcast-satellite (DVB-S2) system (e.g., see European Telecommunications Standards Institute (ETSI) Draft EN 302307, v.1.1.1, June 2004). In DVB-S2, it is known that these LDPC codes have good performance in conjunction with quadrature amplitude modulation (QAM) over a range of QAM schemes such as 4-QAM, 16-QAM, 64-QAM, 256-QAM, 1024-QAM, etc. In each of these QAM schemes, uniform symbol (signal point) constellations are used in DVB-S2 for all LDPC coding rates. Indeed, uniform symbol constellations are even used for DVB-C, DVB-S, and even next generation systems such as DVB-Terrestrial 2 (T2).